A scheme entertained by Arthur Thistlewood and his fellow-conspirators to overthrow the Government by assassinating the Cabinet Ministers. So called from Cato Street, where their meetings were held. (1820.)
The other names of these conspirators are Brunt, Davidson, Harrison, Ings, Monument, Tidd, and Wilson. All eight were sent to the Tower, March 3rd, 1820, by warrant of the Secretary of State.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Cato-Street Conspiracy from Fact Monster: